This invention relates to a pressure swing adsorption process for the selective adsorption of at least one gaseous component from a feed gas mixture, wherein the gaseous mixture is conducted in cyclic alternation through at least three adsorbers which then pass, in staggered fashion with respect to one another, through switching cycles. During an adsorption phase, a feed gas mixture is introduced under elevated pressure into an adsorber and unadsorbed gas is withdrawn from the adsorber. Partial expansion or depressurization of the adsorber in cocurrent mode (i.e., in the same direction as adsorption was performed) occurs after termination of the adsorption phase, and the thus-obtained cocurrent expansion gas is passed on at least in part to other adsorbers to partially repressurize the other adsorbers. After termination of the cocurrent expansion, a countercurrent expansion and, at the lowest process pressure, an optional purge with a purging gas are performed. Finally, the switching cycle is completed by subsequently repressurizing the adsorber to the adsorption pressure.
In conventional pressure swing adsorption processes (also know as PSA processes), multistage cocurrent expansion, i.e., depressurization, takes place normally subsequent to an adsorption phase. The thus-obtained cocurrent expansion gas is conducted into one or several regenerated adsorbers so as to repressurize the regenerated adsorbers. In other words, the cocurrent expansion gas is used to build up pressure within a regenerated adsorber through pressure equalization with the adsorber undergoing a cocurrent expansion phase. Following one or several such pressure equalizations, gas from a further cocurrent expansion phase is utilized in order to purge a further adsorber at the lowest process pressure. The reasonable final pressure level to which a loaded adsorber is brought by cocurrent expansion depends on the concentration of adsorbed components, the adsorption pressure, and the magnitude of adsorbent reserves provided for the advancement of the adsorption front during cocurrent expansion in the adsorber.
In order to attain high product yields, a PSA process known from U.S. Pat. No. 3,986,849 pursues the objective of performing cocurrent expansion to a maximum low pressure level in order to regain much of the product component contained in a gas space of the adsorber. A consequence of this process is that substantially larger adsorbers are needed than are required for the actual adsorption phase. In order to avoid this drawback, a PSA process known from U.S. Pat. No. 4,381,189 terminates the cocurrent expansion at a higher pressure and provides that a portion of the gas obtained during the subsequent countercurrent expansion is additionally purified in auxiliary adsorbers and then reintroduced into the main adsorbers as the purge gas. This means that impurities that had been adsorbed in the main adsorber must be adsorbed a second time in the auxiliary adsorber and thus still involves a high demand for adsorbent.